Growing concerns regarding domestic security have created a critical need to positively identify individuals as legitimate holders of credit cards, driver's licenses, passports and other forms of identification. The ideal identification process is reliable, fast, and relatively inexpensive. It should be based on modern high-speed electronic devices that can be networked to enable fast and effective sharing of information. It should also be compact, portable, and robust for convenient use in a variety of environments, including airport security stations, customs and border crossings, police vehicles, home and office computing and entrance control sites of secure buildings.
A well established method for identification is to compare a fingerprint with a previously obtained authentic fingerprint of the individual. Fingerprints have traditionally been collected by rolling an inked finger on a white paper. Since this traditional process clearly fails to meet the criteria listed above, numerous attempts have been made to adapt an electronically imaged fingerprint method to address new security demands. These modern proposals all use, as a key component, a solid-state device such as a capacitive or optical sensor to capture the fingerprint image in a digital format. By using a new type of solid-state imager as part of a fingerprint identification apparatus a fingerprint can be collected conveniently and rapidly, for example, during a security check, and subsequently correlated, in near real-time, to previously trained digital fingerprints in an electronic data base that resides either in a computer at the security check point, a secure but portable or removable storage device, or on a remotely networked server.
A typical fingerprint comprises a pattern of ridges separated by valleys, and a series of pores that are located along the ridges. The ridges are usually 100 to 300 μm wide and can extend in a swirl-like pattern for several mm to one or more cm. These ridges are separated by valleys with a typical ridge-valley period of approximately 250-500 μm. Pores, roughly circular in cross section, range in diameter from about 60 μm to 240 μm and are aligned along the ridges and can be isolated or grouped into two or more abutting or near abutting pore clusters. There are typically more than 400 pores within a fingerprint region with a frequency of occurrence of about 21 pores/cm of ridge length (see Roddy A. and Stosz J., Proceed; IEEE, 85, 9, 1390-1421 (1997). Almost all present-day fingerprint identification procedures use only ridge/valley minutiae patterns. These are simplified and identified as a pattern of ridge/valley features such as end points, deltoids, bifurcations, crossover points, and islands, all together referred to as minutiae. Typically, a relatively large area of the fingerprint is required in order to obtain enough unique minutiae features, for example, at least 0.50×0.50 inches. Most modern fingerprint imagers therefore use up to one full inch square or even larger, in order to obtain enough features to perform a useful means of identification. Fingerprints are compared using primarily this simplified description of the minutiae patterns.
Due to the more demanding resolution requirements necessary to successfully image pores, there are no commercial devices available today that use pores for fingerprint identification, even though there are typically 7 to 10 ten times as many pores as minutiae in a given fingerprint area. A typical fingerprint image as small as 0.1×0.1 inches may only contain 2-5 minutiae points, not enough to reliably identify a unique individual. The same area, however, may typically contain as many as 40 to 50 pores and several thousand ridge contour details, which along with a few minutiae points can positively identify an individual reliably.
Most optical designs proposed for creating fingerprint images suffer important limitations that reduce their usefulness in real life applications. Many designs are not suitable, for example, to resolve pore patterns or fine detail of the contour of the intersection of ridges and valleys in the fingerprint. Other designs produce distorted images that complicate fingerprint correlation, and still other designs are too bulky or delicate for convenient use in the field.
One optical design that reduces the overall size of the device uses holograms to diffract light in a desired direction. A common limitation of such devices is sensitivity of the intensity of illumination of the target topological surface to variation in temperature with respect to angle and wavelength of the incident light.
Accordingly, there is a need for a compact, high resolution device that reliably operates over a broad range of temperature.